diff --git a/src/jsifier.mjs b/src/jsifier.mjs
index 6d9f25560f09a..f9738f9e88948 100644
--- a/src/jsifier.mjs
+++ b/src/jsifier.mjs
@@ -9,6 +9,7 @@
 
 import assert from 'node:assert';
 import * as fs from 'node:fs/promises';
+import { isAsyncFunction } from 'node:util/types';
 import {
   ATMODULES,
   ATEXITS,
@@ -337,7 +338,7 @@ return ${makeReturn64(await_ + body)};
       return `\
 ${async_}function(${args}) {
 ${argConversions}
-var ret = (() => { ${body} })();
+var ret = (${async_}() => { ${body} })();
 return ${makeReturn64(await_ + 'ret')};
 }`;
     }
@@ -540,13 +541,13 @@ function(${args}) {
         deps.push('setTempRet0');
       }
 
-      let isAsyncFunction = false;
+      let hasAsyncDecorator = false;
       if (ASYNCIFY) {
         const original = LibraryManager.library[symbol];
         if (typeof original == 'function') {
-          isAsyncFunction = LibraryManager.library[symbol + '__async'];
+          hasAsyncDecorator = LibraryManager.library[symbol + '__async'];
         }
-        if (isAsyncFunction) {
+        if (hasAsyncDecorator) {
           asyncFuncs.push(symbol);
         }
       }
@@ -681,6 +682,10 @@ function(${args}) {
         snippet = stringifyWithFunctions(snippet);
         addImplicitDeps(snippet, deps);
       } else if (isFunction) {
+        if (ASYNCIFY == 2 && hasAsyncDecorator && !isAsyncFunction(snippet)) {
+          error(`'${symbol}' is marked with the __async decorator but is not an async JS function.`);
+        }
+
         snippet = processLibraryFunction(snippet, symbol, mangled, deps, isStub);
         addImplicitDeps(snippet, deps);
         if (CHECK_DEPS && !isUserSymbol) {
@@ -775,7 +780,7 @@ function(${args}) {
         }
         contentText += `\n${mangled}.sig = '${sig}';`;
       }
-      if (ASYNCIFY && isAsyncFunction) {
+      if (ASYNCIFY && hasAsyncDecorator) {
         contentText += `\n${mangled}.isAsync = true;`;
       }
       if (isStub) {
diff --git a/src/lib/libasync.js b/src/lib/libasync.js
index c79e4f6bfccbf..3eb5f288f5b9f 100644
--- a/src/lib/libasync.js
+++ b/src/lib/libasync.js
@@ -476,11 +476,11 @@ addToLibrary({
 
   emscripten_sleep__deps: ['$safeSetTimeout'],
   emscripten_sleep__async: true,
-  emscripten_sleep: (ms) => Asyncify.handleSleep((wakeUp) => safeSetTimeout(wakeUp, ms)),
+  emscripten_sleep: async (ms) => Asyncify.handleSleep((wakeUp) => safeSetTimeout(wakeUp, ms)),
 
   emscripten_wget_data__deps: ['$asyncLoad', 'malloc'],
   emscripten_wget_data__async: true,
-  emscripten_wget_data: (url, pbuffer, pnum, perror) => Asyncify.handleAsync(async () => {
+  emscripten_wget_data: async (url, pbuffer, pnum, perror) => Asyncify.handleAsync(async () => {
     /* no need for run dependency, this is async but will not do any prepare etc. step */
     try {
       const byteArray = await asyncLoad(UTF8ToString(url));
@@ -497,7 +497,7 @@ addToLibrary({
 
   emscripten_scan_registers__deps: ['$safeSetTimeout'],
   emscripten_scan_registers__async: true,
-  emscripten_scan_registers: (func) => {
+  emscripten_scan_registers: async (func) => {
     return Asyncify.handleSleep((wakeUp) => {
       // We must first unwind, so things are spilled to the stack. Then while
       // we are pausing we do the actual scan. After that we can resume. Note
@@ -585,7 +585,7 @@ addToLibrary({
 
   emscripten_fiber_swap__deps: ["$Asyncify", "$Fibers", '$stackSave'],
   emscripten_fiber_swap__async: true,
-  emscripten_fiber_swap: (oldFiber, newFiber) => {
+  emscripten_fiber_swap: async (oldFiber, newFiber) => {
     if (ABORT) return;
 #if ASYNCIFY_DEBUG
     dbg('ASYNCIFY/FIBER: swap', oldFiber, '->', newFiber, 'state:', Asyncify.state);
diff --git a/src/lib/libcore.js b/src/lib/libcore.js
index 9c2f828c43920..8fa000cf2eb03 100644
--- a/src/lib/libcore.js
+++ b/src/lib/libcore.js
@@ -2622,7 +2622,7 @@ function wrapSyscallFunction(x, library, isWasi) {
   post = handler + post;
 
   if (pre || post) {
-    t = modifyJSFunction(t, (args, body) => `function (${args}) {\n${pre}${body}${post}}\n`);
+    t = modifyJSFunction(t, (args, body, async_) => `${async_} function (${args}) {\n${pre}${body}${post}}\n`);
   }
 
   library[x] = eval('(' + t + ')');
diff --git a/src/lib/libemval.js b/src/lib/libemval.js
index e9e8553cd49d4..708f8130949a7 100644
--- a/src/lib/libemval.js
+++ b/src/lib/libemval.js
@@ -402,12 +402,19 @@ ${functionBody}
 #if ASYNCIFY
   _emval_await__deps: ['$Emval', '$Asyncify'],
   _emval_await__async: true,
+#if ASYNCIFY == 1
   _emval_await: (promise) => {
     return Asyncify.handleAsync(async () => {
       var value = await Emval.toValue(promise);
       return Emval.toHandle(value);
     });
   },
+#else
+  _emval_await: async (promise) => {
+    var value = await Emval.toValue(promise);
+    return Emval.toHandle(value);
+  },
+#endif
 #endif
 
   _emval_iter_begin__deps: ['$Emval'],
diff --git a/src/lib/libidbstore.js b/src/lib/libidbstore.js
index 224d1d1d434a8..a66809290f7c0 100644
--- a/src/lib/libidbstore.js
+++ b/src/lib/libidbstore.js
@@ -94,7 +94,7 @@ var LibraryIDBStore = {
 #if ASYNCIFY
   emscripten_idb_load__async: true,
   emscripten_idb_load__deps: ['malloc'],
-  emscripten_idb_load: (db, id, pbuffer, pnum, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_load: async (db, id, pbuffer, pnum, perror) => Asyncify.handleSleep((wakeUp) => {
     IDBStore.getFile(UTF8ToString(db), UTF8ToString(id), (error, byteArray) => {
       if (error) {
         {{{ makeSetValue('perror', 0, '1', 'i32') }}};
@@ -110,7 +110,7 @@ var LibraryIDBStore = {
     });
   }),
   emscripten_idb_store__async: true,
-  emscripten_idb_store: (db, id, ptr, num, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_store: async (db, id, ptr, num, perror) => Asyncify.handleSleep((wakeUp) => {
     IDBStore.setFile(UTF8ToString(db), UTF8ToString(id), new Uint8Array(HEAPU8.subarray(ptr, ptr+num)), (error) => {
       // Closure warns about storing booleans in TypedArrays.
       /** @suppress{checkTypes} */
@@ -119,7 +119,7 @@ var LibraryIDBStore = {
     });
   }),
   emscripten_idb_delete__async: true,
-  emscripten_idb_delete: (db, id, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_delete: async (db, id, perror) => Asyncify.handleSleep((wakeUp) => {
     IDBStore.deleteFile(UTF8ToString(db), UTF8ToString(id), (error) => {
       /** @suppress{checkTypes} */
       {{{ makeSetValue('perror', 0, '!!error', 'i32') }}};
@@ -127,7 +127,7 @@ var LibraryIDBStore = {
     });
   }),
   emscripten_idb_exists__async: true,
-  emscripten_idb_exists: (db, id, pexists, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_exists: async (db, id, pexists, perror) => Asyncify.handleSleep((wakeUp) => {
     IDBStore.existsFile(UTF8ToString(db), UTF8ToString(id), (error, exists) => {
       /** @suppress{checkTypes} */
       {{{ makeSetValue('pexists', 0, '!!exists', 'i32') }}};
@@ -137,7 +137,7 @@ var LibraryIDBStore = {
     });
   }),
   emscripten_idb_clear__async: true,
-  emscripten_idb_clear: (db, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_clear: async (db, perror) => Asyncify.handleSleep((wakeUp) => {
     IDBStore.clearStore(UTF8ToString(db), (error) => {
       /** @suppress{checkTypes} */
       {{{ makeSetValue('perror', 0, '!!error', 'i32') }}};
@@ -146,7 +146,7 @@ var LibraryIDBStore = {
   }),
   // extra worker methods - proxied
   emscripten_idb_load_blob__async: true,
-  emscripten_idb_load_blob: (db, id, pblob, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_load_blob: async (db, id, pblob, perror) => Asyncify.handleSleep((wakeUp) => {
 #if ASSERTIONS
     assert(!IDBStore.pending);
 #endif
@@ -174,7 +174,7 @@ var LibraryIDBStore = {
     });
   }),
   emscripten_idb_store_blob__async: true,
-  emscripten_idb_store_blob: (db, id, ptr, num, perror) => Asyncify.handleSleep((wakeUp) => {
+  emscripten_idb_store_blob: async (db, id, ptr, num, perror) => Asyncify.handleSleep((wakeUp) => {
 #if ASSERTIONS
     assert(!IDBStore.pending);
 #endif
diff --git a/src/lib/libpromise.js b/src/lib/libpromise.js
index db4838e09cc22..0be6872d09c52 100644
--- a/src/lib/libpromise.js
+++ b/src/lib/libpromise.js
@@ -261,7 +261,7 @@ addToLibrary({
 #if ASYNCIFY
   emscripten_promise_await__deps: ['$getPromise', '$setPromiseResult'],
 #endif
-  emscripten_promise_await: (returnValuePtr, id) => {
+  emscripten_promise_await: async (returnValuePtr, id) => {
 #if ASYNCIFY
 #if RUNTIME_DEBUG
     dbg(`emscripten_promise_await: ${id}`);
diff --git a/src/lib/libsdl.js b/src/lib/libsdl.js
index c810a2c5e0302..f380329b0dec8 100644
--- a/src/lib/libsdl.js
+++ b/src/lib/libsdl.js
@@ -1746,7 +1746,7 @@ var LibrarySDL = {
 #if ASYNCIFY
   SDL_Delay__deps: ['emscripten_sleep'],
   SDL_Delay__async: true,
-  SDL_Delay: (delay) => _emscripten_sleep(delay),
+  SDL_Delay: async (delay) => _emscripten_sleep(delay),
 #else
   SDL_Delay: (delay) => {
     if (!ENVIRONMENT_IS_WORKER) abort('SDL_Delay called on the main thread! Potential infinite loop, quitting. (consider building with async support like ASYNCIFY)');
diff --git a/src/lib/libwasi.js b/src/lib/libwasi.js
index 2d3b143f0969a..316a189ba9c38 100644
--- a/src/lib/libwasi.js
+++ b/src/lib/libwasi.js
@@ -532,7 +532,7 @@ var WasiLibrary = {
     return 0;
   },
 
-  fd_sync: (fd) => {
+  fd_sync: {{{ asyncIf(ASYNCIFY) }}} (fd) => {
 #if SYSCALLS_REQUIRE_FILESYSTEM
     var stream = SYSCALLS.getStreamFromFD(fd);
 #if ASYNCIFY
diff --git a/test/codesize/test_codesize_hello_dylink_all.json b/test/codesize/test_codesize_hello_dylink_all.json
index f07e4dfa5ee9d..346c9fb9c43b9 100644
--- a/test/codesize/test_codesize_hello_dylink_all.json
+++ b/test/codesize/test_codesize_hello_dylink_all.json
@@ -1,7 +1,7 @@
 {
-  "a.out.js": 245841,
+  "a.out.js": 245846,
   "a.out.nodebug.wasm": 597746,
-  "total": 843587,
+  "total": 843592,
   "sent": [
     "IMG_Init",
     "IMG_Load",
diff --git a/test/test_browser.py b/test/test_browser.py
index 7b8a868ada5f6..923c86b883053 100644
--- a/test/test_browser.py
+++ b/test/test_browser.py
@@ -4982,8 +4982,6 @@ def test_embind_with_pthreads(self):
   def test_embind(self, args):
     if is_jspi(args) and not is_chrome():
       self.skipTest(f'Current browser ({common.EMTEST_BROWSER}) does not support JSPI. Only chromium-based browsers ({CHROMIUM_BASED_BROWSERS}) support JSPI today.')
-    if is_jspi(args) and self.is_wasm64():
-      self.skipTest('_emval_await fails')
 
     self.btest('embind_with_asyncify.cpp', '1', cflags=['-lembind'] + args)
 
diff --git a/test/test_other.py b/test/test_other.py
index 8caf035a2f2b6..cb1afa722a2f4 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -3242,10 +3242,11 @@ def test_embind_asyncify(self):
     ''')
     self.do_runf('main.cpp', 'done', cflags=['-lembind', '-sASYNCIFY', '--post-js', 'post.js'])
 
+  @also_with_wasm64
   @parameterized({
-    '': [['-sDYNAMIC_EXECUTION=1']],
-    'no_dynamic': [['-sDYNAMIC_EXECUTION=0']],
-    'dyncall': [['-sALLOW_MEMORY_GROWTH', '-sMAXIMUM_MEMORY=4GB']],
+    '': (['-sDYNAMIC_EXECUTION=1'],),
+    'no_dynamic': (['-sDYNAMIC_EXECUTION=0'],),
+    'dyncall': (['-sALLOW_MEMORY_GROWTH', '-sMAXIMUM_MEMORY=4GB'],),
   })
   @requires_jspi
   def test_embind_jspi(self, args):
@@ -5012,6 +5013,24 @@ def test_jslib_system_lib_name(self):
 ''')
     self.do_runf('src.c', 'jslibfunc: 12', cflags=['--js-library', 'libcore.js'])
 
+  @requires_jspi
+  def test_jslib_jspi_missing_async(self):
+    create_file('lib.js', r'''
+      addToLibrary({
+        foo__async: true,
+        foo: function(f) {},
+      });
+    ''')
+    create_file('main.c', r'''
+      #include <emscripten.h>
+      extern void foo();
+      EMSCRIPTEN_KEEPALIVE void test() {
+        foo();
+      }
+    ''')
+    err = self.expect_fail([EMCC, 'main.c', '-o', 'out.js', '-sJSPI', '--js-library=lib.js', '-Wno-experimental'])
+    self.assertContained('error: \'foo\' is marked with the __async decorator but is not an async JS function.', err)
+
   def test_EMCC_BUILD_DIR(self):
     # EMCC_BUILD_DIR was necessary in the past since we used to force the cwd to be src/ for
     # technical reasons.